Abstract
The lumbrical-interossei comparison method (2L-INT method) is recommended for the diagnosis of carpal tunnel syndrome owing to its high sensitivity and simplicity. However, it is not easy to find the second lumbricalis (2L) on the palm surface to obtain the compound muscle action potential (CMAP). In this study, therefore, we explored a new approach to locating the proper electrode position in the 2L-INT method. We drew a line starting from the joint between the second finger and the third finger to the carpal center, and on this line, we determined point “a” at the metacarpal head height, point “d” at the boundary between this line and the thenar muscle, and points “b” and “c” respectively at the first and second one-third points on the line from “a” to “d”. We then examined, at each electrode position (a, b, c, and d) the following: (1) latency, (2) CMAP amplitude, and (3) CMAP rising time slopes of both 2L-CMAP and INT-CMAP. We also examined (4) the 2L-INT latency difference and (5) the different results obtained by different examiners. At points “a” and “d,” low amplitude and positive rising time slopes were observed occasionally. The 2L-INT time was also extended in some cases, possibly causing a false positive result. Thus, points “a” and “d” seemed unsuitable for an active electrode. In contrast, at the midpoint between points “b” and “c,” the amplitude was high, and there were few changes in the 2L-INT time caused by the electrode position differences. Thus, this location seemed suitable for an active electrode.
